"Halt die Fresse" is more or less "Shut (your fucking) face", just not quite as aggressive. Meanwhile, "fressen" as a verb also means to scarf down food, devour hastily, eat like a pig, you know the drill. Technically it serves to describe the way animals eat, whereas "essen" literally means "eat" and is used when dealing with humans. As it stands, you don't have to fear about being all that rude when using "fressen" as a word. If anything, you would maybe be considered uncouth. Though even that isn't necessarily like that. Really depends on the situation and who you deal with.